The occurrence of both rheumatic fever and poststreptococcal glomerulonephritis in the island population of Trinidad offers a unique opportunity to study these two diseases simultaneously in the same population. Our particular interest has focused on the possibility that cellular reactivity to streptococcal antigens in rheumatic fever patients (perhaps genetically determined) may play a role in the disease process. Using two in vitro parameters of cellular reactivity (direct capillary migration inhibition of leucocytes and blastogenic transformation in the presence of streptococcal antigens), our results show that rheumatic individuals react primarily to streptococcal membrane antigens and only from those strains which are associated with rheumatic fever on the island. Our present efforts are to further purify and isolate those fractions responsible for this unique reactivity. In a continuing search for possible genetic markers in these rheumatic individuals, recent studies with a single B cell alloantiserum indicate that approximately 75% of all rheumatic fever patients contain this antigen on the surface of their B lymphocytes. These results strongly suggest that rheumatic individuals share common genetic markers on their cells and may have potential importance regarding pathogenesis of the disease.